1. Field of the Invention
Lithography is a widely used process in the field of commercial printing, for the production of such items as newspapers, magazines, advertising brochures and packaging materials. The quality of printing, primarily four-color printing, has steadily improved due to advances in the quality of inks, papers and printing machinery. In addition to improved mechanical designs in printing presses, automatic means of monitoring and controlling various aspects of press operation have contributed to improved printing quality.
Offset lithographic printing involves the transfer of ink via a set of rollers to image areas of a printing plate, which are oleophilic. From these image areas, ink is transferred to a rubber cylinder called a blanket, which then applies the ink to paper or other substrates. In addition to ink, a hydrophilic fountain solution, composed mainly of water, is applied via a set of rollers to the hydrophilic non-image areas of the printing plate. The fountain solution keeps the ink from covering the non-image area. In addition to water, the fountain solution often contains iso-propanol or surfactants similar in composition to soap, as well as other possible ingredients. Fountain solution is often referred to simply as "water".
It should not be assumed from the previous statements that the fountain solution and ink do not mix, or even that their mixing is undesirable. In fact, practical experience shows that proper printing requires that some fountain solution be emulsified in the ink. If too little fountain solution is in the ink, the ink can spread into the non-image areas. This is called plate "catch-up. In addition, too little water can increase ink tack, causing "picking", a removal of part of the surface of the printed page. Thus, a proper "ink-water balance" is desired.
Emulsification of ink in water is caused by the extremely high pressure in the nip between the ink form roller, which is coated with ink, and the lithographic plate, which is coated with fountain solution in the nonimage areas prior to contact with the ink roller. In Dahlgren dampening systems, fountain solution is applied directly to the ink form roller. Further emulsification arises in the form roller-distributor nip.
The degree of emulsification of the fountain solution in ink is a function of the fountain solution itself, the particular ink, press roller speed, number of rollers, pressure between rollers, ambient temperature, humidity and numerous other variables. Also, it is necessary to distinguish between the total fountain solution in contact with ink and that actually emulsified in ink. Fountain solution located on the surface of the ink does not alter the rheological properties of the ink as does emulsified solution. As the printing press runs, many of the variables described above will change, causing the interaction between ink and water to change.
2. Description of the Prior Art
An automatic instrumental method of monitoring the fountain solution and its interaction with ink is desirable in light of the importance of proper ink-water balance. Prior attempts to monitor ink-water balance include D. L. Southam's (U.S. Pat. No. 3,499,383) and P. R. Kantor's (U.S. Pat. No. 3,412,677) determination off fountain solution level by electrical conduction measurements or D. K. Mikan and A. A. Presti's (U.S. Pat. No. 3,822,643) determination by impedance measurements on water layers in roller nips using auxiliary rollers. A related approach by W. E. Dauterman (U.S. Pat. No. 3,730,086) measures the capacitance of a layer of fountain solution between an auxiliary roller and a concentric capacitor plate. These methods require modification of the press, determine the amount of fountain solution only and do not distinguish between surface and emulsified solution. The use of gloss measurements on the printing plate, as described by J. Albrecht, W. Rebner and B. Wirz (Forschungsbericht Land Nordheim Westfalen No. 1523 Westdeutscher Verlag, Cologne, 1966) or the use of infra-red absorption measurements of water on the printing plate, as described by B. Wirz, R. Bosse, P. Decker and D. Pyliotis (Fogra Institutsmitteilung 3202/3203, Munich 1972) are used to determine only the amount of fountain solution in the former case and water in the latter. Neither method determines both surface and emulsified solution. Both techniques require changes in the position of the light source and detector to accomodate the position of the non-image areas, which will differ from plate to plate. Reflectance measurements with the plate as a substrate are subject to interference patterns arising from thin oxide layers on the plate, and can vary as the plate wears. Interference fringes also arise due to the thinness of the layer of solution on the plate (typically&lt;5 micrometers). Furthermore, the position and intensity of the absorbance peak due to the hydroxyl group in water is subject to the nature and concentration of the fountain solution additives. S. Karttunen and M. Ilvessuo (NATS Research Report, Graphic Arts Research Institute, Otaniemi, 1975) and K. Reich (Research Report, IGT Leipzig, 1964) describe a method of gravimetrically determining the amount of water, off line, and without distinguishing surface and emulsified water. Karl Fischer titration for determination of amount of water or T. Saynevirta and S. Karttunen's (Graphic Arts in Finland 2 (1973) 2, pp. 1-12) method of doping the solution with a radioactive tracer to determine the amount of solution both determine only water or solution content, do not distinguish between surface and emulsified water and are each off-line methods. D. Pyliotis's approach of taking infra-red measurements of the ink and water using an auxiliary roller (Fogra Forschungsberich No. 5 205, 1978) and J. Albrecht and M. Heigl's technique of measuring the change in dielectric constant of ink as a function of added solution (Fogra Mitteilungen 14 (1965), 47, pp. 3-9) require modification of the press and do not distinguish between surface and emulsified solution. G. W. Jorgensen's (U.S. Pat. No. 3,191,528) approach of attaching a tackmeter to an ink roller and measuring ink tack, requires contact between the meter and the roller and, of course, measures only tack.